Japan`s Kei Nishikori withdrew on Sunday from next week`s ATP and WTA Cincinnati Masters tournament with a left hip injury.
The fourth seed lost a semi-final in Montreal at the weekend after winning the Washington title a week earlier.
"I kind of got hurt last week in Montreal," said the 2014 US Open finalist. "I played five matches in Washington and played four matches in Montreal.
"My hip was bothering me so I couldn`t really play and could barely finish the match. It`s very unfortunate to pull out here, but I had a good two tournaments so I`m very happy with my tennis.
"My leg was sore the last couple of days. Yesterday it got a little bit worse."
Asia`s top player said that he would take a rest prior to the start of the US Open which begins August 31.
"I`m sure it`s going to be OK. I think it`s going to take one week or so. I`m going to try to get good recovery here this week and I hope I can play good again in the US Open."